An internal combustion engine is known in which a three-way catalyst is disposed in an engine exhaust passage, an NOx adsorption catalyst adsorbing NOx in exhaust gas when an air-fuel ratio of the introduced exhaust gas is in a lean range and discharging the adsorbed NOx when the air-fuel ratio of the introduced exhaust gas is in a rich range is disposed in the engine exhaust passage downstream of the three-way catalyst, and an engine operation mode is switched to either a lean air-fuel ratio operation mode in which combustion is carried out at a lean air-fuel ratio or a theoretical air-fuel ratio operation mode in which combustion is carried out at a theoretical air-fuel ratio depending on engine operation states (for example, see Patent Literature 1).
In such an internal combustion engine, the amount of fuel consumed when the combustion is carried out at the lean air-fuel ratio is less than that when the combustion is carried out at the theoretical air-fuel ratio. Accordingly, in the internal combustion engine, the combustion is generally carried out at the lean air-fuel ratio in an operation area that is as wide as possible. However, when the combustion is carried out at the lean air-fuel ratio in a state in which an engine load is high, the temperature of the NOx adsorption catalyst rises and thus NOx adsorption capability of the NOx adsorption catalyst decreases, whereby an NOx purification rate decreases. Accordingly, in the internal combustion engine, the operation mode is switched from the lean air-fuel ratio operation mode to the theoretical air-fuel operation mode when the engine load becomes high so as not to decrease the NOx purification rate.